The present invention relates to UV absorbing arc tube bodies for use in electric lamps. It finds particular application in conjunction with ceramic metal halide arc tubes, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to other lamp envelopes and shrouds for lamps having a source of light which emits both UV and visible light radiation, such as those constructed of single crystal sapphire or quartz.
Metal halide lamps are typically constructed of a fused silica (quartz) arc tube containing a fill of a light emitting metal, such as sodium, commonly in the form of the halide, and optionally mercury. The lifetime of such lamps is often limited by the loss of the metal portion of the metal halide fill during lamp operation due to metal ion diffusion, or reaction of the metal halide with the fused silica arc tube, and a corresponding build-up of free halogen in the arc tube.
The mobility of the sodium is such that the quartz arc tubes are relatively porous to it. During lamp operation, sodium passes from the hot plasma and through the arc tube wall to the cooler region between the arc tube and the outer jacket or envelope. The lost sodium is thus unavailable to the discharge and can no longer contribute its characteristic emission. The light output consequently diminishes and the color shifts from white toward blue. The arc becomes constricted and, particularly in a horizontally operating lamp, may bow against the arc tube wall and soften it. Also, loss of sodium causes the operating voltage of the lamp to increase and it may rise to the point where the arc can no longer be sustained, ending the life of the lamp.
Ceramic arc tubes have recently been developed which provide advantages over quartz arc tubes. Ceramic arc tubes are formed from densely sintered polycrystalline metal oxide, such as aluminum oxide, yttrium-aluminum garnet, or yttrium oxide, or a monocrystalline metal oxide, such as sapphire. U.S. Patent Nos. 5,424,609; 5,698,948; and 5,751,111 provide examples of such arc tubes. Ceramic arc tubes are less porous to sodium ions than quartz and thus retain the metal within the lamp. They are also able to withstand much higher operating temperatures than quartz arc tubes. While quartz arc tubes are limited to operating temperatures of around 950-1000.degree. C., due to reaction of the halide fill with the glass, ceramic arc tubes are capable of withstanding operating temperatures of 1700 to 1900.degree. C. The higher operating temperatures provide better color rendering and higher lamp efficiency.
Ceramic arc tubes are constructed of a number of separate parts which are extruded or die pressed from a ceramic powder, mixed with an organic binder. European patent Application No. 0 587 238 A1, for example, discloses a ceramic discharge tube of translucent aluminum oxide. Typically, the parts are tacked together with an adhesive and then sintered to form cohesive gas-tight joints between the components.
Both quartz and ceramic electric arc discharge lamps having mercury in the arc discharge emit ultraviolet (UV) radiation which at wavelengths below 320 nm are harmful to the human eye, fabrics, plastics and other materials. One way of reducing UV in metal halide lamps is to surround the arc tube with a fixture having a UV absorbing lens or an outer glass jacket which contains UV absorbing materials, a costly additional step.
Dopants have been used in quartz lamps to absorb UV radiation emitted by the light source. Examples are disclosed in U.S. Pat. Nos. 2,582,453, 2,862,131, and 3,148,300. Many of the UV absorbers used in the quartz glass result in a color change. For example, U.S. Pat. No. 2,862,131 discloses green or brown tinted glass.
The present invention provides a new and improved UV-absorbing arc tube body which overcomes the above referenced problems and others.